This invention relates to a streak image sensor and a method of operating a streak image sensor.
The word “submerged” as used in this application as applied to an object in a body of water means both fully submerged, so that the object does not project above the free surface of the water, and partially submerged, so that the object projects partially above the free surface of the water.
It is desirable to identify and locate submerged objects, such as flotsam and jetsam, in order to mitigate the danger that such objects may pose to navigation. In addition, it may be desirable to monitor the location of objects that have been deliberately deployed at or below the water surface, such as equipment for recovery of wave energy in order to check the equipment for damage.
Airborne light detection and ranging (LIDAR) systems have been used to obtain information regarding depth and location of submerged objects. FIG. 1 illustrates schematically one possible implementation of an airborne LIDAR system. As shown in FIGS. 1A and 1B, a probe 2 (which may be suspended from an aircraft traveling in a direction X) contains a laser light source 4 that emits a pulsed beam of light directed downward towards the water surface 6. The pulse emitted by the laser source 4 may have a duration of about 6 ns full width at half maximum amplitude. Suitable optics (not shown) spread the narrow output beam of the laser light source to create a fan beam 7 (see FIG. 1B). The fan beam 7 illuminates a strip of the water surface that extends perpendicular to the direction of travel of the probe. Some of the laser light may be reflected from the water surface, some light that enters the water is absorbed, some light that enters the water is backscattered by the water, and some light is reflected off objects 8 that are more or less close to the water surface. Some of the reflected and backscattered light is returned to the probe as a return laser light signal. The time at which return light (reflected and backscattered) is received from a depth d below the water surface depends on the depth d and on the height h of the probe above the water surface. The return light varies in intensity as a function of time depending on whether the return light is reflected or backscattered, the reflective properties of reflective surfaces, the shape and orientation of reflective surfaces, the optics of the transmitter and receiver, the distance traveled by the return light, and the attenuation and scattering of the light.
The return light is delivered to a streak camera 10, having multiple sensor channels CH1-CHn (FIG. 2) each including a sensor element 12, such as an avalanche photodiode (APD). Each channel CH receives return light along a respective azimuth θ (relative to vertical) of the fan beam from an instantaneous field of view (IFOV) of its sensor element. The IFOV of each sensor element contains a respective segment of the illuminated strip of the water surface. The IFOV of each sensor element 12 is stationary relative to the LIDAR system itself but moves relative to a terrestrial frame of reference as the aircraft moves, and accordingly, the instantaneous terrestrial field of view of each sensor element changes as the aircraft moves along its trajectory.
The current signals generated by the APDs 12 are amplified by a preamplifier 14 and converted to respective voltage signals which are sampled and stored using a switched capacitor array (SCA) 16. As shown in FIG. 2, the capacitors are organized in a rectangular array of n rows and m columns where each row i (i=1 . . . n) is associated with one channel of the streak camera and each column j (j=1 . . . m) spans all channels and contains one capacitor of each channel. Each capacitor Cxj (j=1 . . . m) in row x is selectively connected to the output of the channel preamplifier 14x by closing (rendering conductive) and then opening (rendering non-conductive) a sampling switch Sxj. The sampling interval between closing two consecutive sampling switches in row x may be about 2-10 ns, corresponding to a resolution of about 0.25-1.25 m in water. The sampling switches S in each row of the SCA are controlled so that the n switches Siy in column y close essentially simultaneously and open essentially simultaneously.
Subsequently, the voltages stored on the capacitors are read out and exported for evaluation by sequentially closing readout switches Rij. The readout switches R may be operated either in column sequential fashion and the output signals multiplexed to a common ADC for digitizing and subsequent processing or the switches of the m columns may be operated synchronously and the output signals supplied to respective ADCs for digitizing and processing. The output signals may be used to drive a display in which the brightness of the display depends on the amplitude of the signal. In this manner, a 2D range-azimuth image of the return light received in response to each laser pulse is obtained. Examination of the images obtained in response to successive laser pulses provides information regarding the depth and location of submerged objects.
It has been recognized that the 2D image that is acquired in the manner described above is distorted because the arrival time of the return light depends on the azimuth angle of the light path relative to vertical. Thus, referring to FIG. 1B, the round-trip propagation time of return light received from the water surface along a path at an angle θ to vertical by a probe at altitude h is 2*h/c*cos θ, where c is the velocity of light in air. In addition, local conditions may result in the round-trip propagation time varying as a function of azimuth angle of the light path. For example, when viewing a river or other narrow body of water having overhanging trees along one or both banks, reflections from the foliage will be received before return light received from the water surface beneath the foliage.